Industrial processing and manufacturing applications require the use of highly toxic fluids. The manufacture of semiconductor materials represents one such application wherein the safe storage and handling of highly toxic hydride or halide gases and mixtures thereof becomes necessary. Examples of such gases include silane, germane, ammonia, phosphine, arsine, boron trifluoride, stibine, hydrogen sulfide, hydrogen selenide, hydrogen telluride, phosphorous trifluoride, arsenic pentafluoride and other halide or hydride compounds and gas mixtures thereof. As a result of toxicity and safety considerations, these gases must be carefully stored and handled in the industrial process facility. The semiconductor industry in particular relies on various gaseous sources, such as, for example, arsine (AsH3) and phosphine (PH3), hydrogen selenide (H2Se), boron triflouride (BF3), diborane (B2H6), silicon tetrafluoride (SiF4), germanium tetrafluoride (GeF4), selenium hexafluoride (SeF6), carbon monoxide (CO) and carbon dioxide (CO2) as sources of arsenic (As), phosphorus (P), boron (B), silicon (Si), germanium (Ge), selenium (Se) and carbon (C) in ion implantation. Ion implantation systems typically use pure gases such as AsH3 and PH3 stored as liquefied compressed gases at their respective vapor pressures and pure gases such as BF3 and SiF4 stored at pressures as high as 1500 psig within the delivery vessel. Due to their extreme toxicity and high vapor pressure, their use, transportation and storage raise significant safety concerns for the semiconductor industry.
To address the various safety concerns, there have been a number of systems developed to deliver these hydride and halide compounds to the ion implant tool at sub-atmospheric conditions. End-users flow rates typically will range from about 0.1-10 sccm. Device safety requires delivery of the gas at sub-atmospheric pressures so that if the valve opens to atmosphere, nothing will leak out of cylinder. A vacuum condition must be applied to the cylinder to obtain flow of gas. As such, a fail-safe vacuum-actuated valve design is required.
For example, a chemical system, known as SDS™ and commercialized by ATMI, Inc. involves filling a compressed gas cylinder with a physical adsorbent material, and reversibly adsorbing the dopant gases onto the material. The desorption process involves applying a vacuum or heat to the adsorbent material/cylinder. In practice, vacuum from the ion implanter is used to desorb the gas from the solid-phase adsorbent. There are certain limitations associated with the SDS technology, and they include the following: 1) the adsorbent material has a finite loading capacity thereby limiting the amount of product available in a given size cylinder; 2) the desorption process can be initiated by exposing the cylinder package to heat, thereby causing the cylinders to reach and deliver gases at atmospheric and super-atmospheric pressures when the cylinder is exposed to temperatures greater than 70° F., which are common in many cylinder warehouse locations and within the ion implant tool; 3) the purity of the gas delivered from the cylinder can be compromised due to adsorption/desorption of the other materials/gases on the adsorbent material; 4) cylinder utilization is influenced by the depth of vacuum applied to the package, such that cylinders can often be returned with appreciable unused product remaining in the package; and 5) adsorbent attrition can lead to particulate contamination in the gas delivery system.
Separately, a number of mechanical systems have been developed for the sub-atmospheric delivery of dopant gases. Some involve the use of a pressure regulator, while others require valve devices to control and deliver the product sub-atmospherically. These devices are set to deliver or open when sub-atmospheric or vacuum conditions are applied to the delivery port of the cylinder. The exact location of these devices can be in the port body, in the neck cavity, or inside the cylinder itself. In each case, the pressure regulator or valve device is located upstream of the cylinder valve seat with respect to flow of gas from the interior of the cylinder to the delivery port.
U.S. Pat. Nos. 6,089,027 and 6,101,816 are both related to a fluid storage and dispensing system comprising a vessel for holding a desired pressure. The vessel contains a pressure regulator, e.g., a single-stage or multi-stage regulator, associated with a port of the vessel, and set at a predetermined pressure. A dispensing assembly, e.g., including a flow control means such as a valve, is arranged in gas/vapor flow communication with the regulator, whereby the opening of the valve effects dispensing of gas/vapor from the vessel. The fluid in the vessel may be constituted by a liquid that is confined in the vessel at a pressure in excess of its liquefaction pressure at prevailing temperature conditions, e.g., ambient (room) temperature.
U.S. Pat. No. 6,857,447 B2 discloses a gas dispensing assembly wherein the source vessel contains a gas at pressures ranging from 20 to 2,000 psig. The apparatus requires a high pressure gas cylinder with a larger than typical neck opening to accommodate the introduction of two pressure regulators in series along the fluid discharge path. The first regulator on the inlet gas side drops the pressure from 1,000 psig (or the actual pressure within the vessel at the time) to 100 psig, while the second regulator from 100 psig to sub-atmospheric pressure.
U.S. Pat. No. 7,905,247 discloses a fluid storage and dispensing vessel having a dispensing vacuum-actuated valve and a flow restriction arrangement to provide a virtually fail safe system for preventing hazardous discharge of fluid from a pressurized cylinder or tank. The valve element includes a poppet valve 32 and a pin 42 seated against the poppet valve 32, as depicted at FIG. 3. The poppet valve 32 is a conical shaped pin device 42 which fits into a matching conical seating cavity referred to as valve seat 36. An elastomer material is molded onto the poppet valve 32. In the closed condition, spring 34 normally presses poppet valve 32 against a valve seat 36. When the valve 32 opens in response to a predetermined vacuum condition surrounding the chamber of the bellows 38 which is internally sealed at atmospheric pressure or higher, bellows 38 expands in the longitudinal direction to create a downward force that is sufficient to urge the poppet valve 32 downwards, thereby moving the poppet valve 32 away from seat 36, which creates a gap for pressurized gas within the interior of cylinder to flow therethrough. U.S. Pat. Nos. 6,007,609 and 6,045,115 disclose flow restrictors disposed along the fluid flow path and which provide capillary size openings that minimize any discharge of toxic gases from compressed gas cylinders in the unlikely event that the dispensing valve fails.
An alternative vacuum-actuated valve design is an o-ring design as shown in FIG. 2 of U.S. Pat. No. 7,905,247, which discloses an o-ring 88 disposed concentrically around the lower base section of a pin 90 that is held in place by a spring bushing. The pin 43 is adapted to move between a sealing position that blocks fluid flow along a fluid flow path and an open position that permits fluid along the fluid flow path. Specifically, a recessed counterbore or groove is provided within a valve base 84 to accommodate an o-ring 88 which is generally formed from a perfluro-elastomer material that is designed to seal as well as compress and decompress. The o-ring 88 remains in a stationary position within the groove of the valve base 84. When the valve is in the closed position, the o-ring 88 is compressed within the groove. The compression of the o-ring 88 between the valve base 84 and the base 90 of the pin 43 prevents the flow of the gas. In the open position, the bellows chamber 50, which is sealed at atmospheric pressure or higher, expands in response to a predetermined vacuum condition surrounding the bellows chamber 50 to create a downward force that is sufficient to urge the stem portion of the pin 43 downwards, thereby moving the base 90 of the pin 43 away from the o-ring 88, which creates a gap for pressurized gas within the interior of cylinder to flow therethrough.
Despite the related storage and delivery systems, sub-atmospheric flow instability continues to be a significant problem. In particular, delivery pressure oscillations and opening pressure spikes persist with all of the above mentioned vacuum-actuated valve and regulator designs. Ion implant tools tend to be sensitive to such delivery pressure spikes and oscillations, as the flow instability creates flow spikes which can automatically shut down an ion implant tool at a semiconductor fabrication plant, thereby resulting in increased down time for the semiconductor manufacturer. Additionally, there are instances when the pin-poppet valve design problematically does not dispense gas at desired downstream vacuum conditions.
In view of such drawbacks, there remains a need for an improved vacuum-actuated valve assembly capable of flow stability. Other objects and aspects of the present invention will become apparent to one of ordinary skill in the art upon review of the specification, drawings and claims appended hereto.